In copending application Ser. No. 149,291, filed May 12, 1980 by P. Casper et al entitled Repeatered, Multi-Channel Fiber Optic Communication Network Having Fault Isolation System, assigned to the assignee of the present application, there is described a high data rate digital signalling environment wherein the transmission of high density signalling traffic, such as T-4 telephone trunk voice signals and data, is carried out over a plurality of fiber optic transmission channels between geographically relatively remote signal interfacing terminal stations. The terminal stations may be coupled to receive the signals to be transmitted from trunk interface port connections, microwave interface equipment, etc., and include electro-optic transceiver equipment for coupling multi-megabit digital data signals over a fiber optic communication network. At these stations processor-based control equipment is provided which serves to perform control and monitoring functions that govern the overall operation of the network. As is explained in the above-referenced application, the network includes protection equipment that serves to maximize the integrity of each channel, the protection equipment being substituted in place of a normally used fiber optic link in the event of unacceptable signal degradation or channel failure. The control and operation of this protection equipment as well as the control and operation of other portions of the system may be effected internally by way of the processor-based monitor and control equipment. These actions may also be carried out externally through operator intervention, by-passing an internally programmed control sequence when necessary.
Of course, as will be readily appreciated by those skilled in the art, it is common practice in sophisticated communication networks to employ operator command consoles through which monitor and control actions may be effected. In an environment where separate and remote facilities are provided for this purpose, such as in the environment of the network described in the above-identified application, some means of providing a signalling link, preferably a voice link, between the station operators is required in order that system supervisory personnel can communicate directly with each other and take whatever effective action is necessary to enable the network to perform as intended.
A common expedient for providing this voice signalling capability is a conventional telephone communication system through which the network operators call one another just as do other subscribers of the system. Unfortunately, this approach subjects the intended voice link between stations to the performance of the external telephone equipment, including circuit availability and channel integrity. In a fiber optic communication network such as that described in the above-referenced application, a major feature of the network is its use for conveying high density telephone trunk traffic between relatively remote stations over a plurality of fiber optic channels. If, in such a network, voice communications between station operators were to be conducted through external telephone equipment whose signals, in turn, were formatted for transmission over the very network being monitored and controlled by the station operators, it can be seen that serious consequences could result from a fault on the channel over which the voice signals for operator-to-operator communications were being conducted.